1. Field of the Invention
The present invention relates to an AC variable speed driving apparatus and an electric vehicle using the same. The electric vehicle generally employs a battery as a power supply and an inverter for converting the DC voltage of the battery to the AC voltage to be supplied to its driving apparatus.
2. Description of the Related Art
FIG. 1 is an electric system for an electric vehicle known in the art, which comprises a battery as a power supply, and drives wheels by AC motors via inverters. In FIG. 1, the reference numeral 1 designates a battery comprising a sufficient number of unit cells 100 connected in series. The reference numeral 4 denotes an inverter that supplies AC power to an AC motor 5 for driving wheels 81 and 82. The output shaft of the motor 5 is coupled to a differential gear 7 via a reduction gear 6, and drives the wheels 81 and 82. A protective fuse 3 is used as needed and a main switch 2 connects or disconnects the battery 1 to or from the inverter 4.
The performance of an electric vehicle must be comparable to that of an internal combustion engine car.
FIG. 2 illustrates an example of the torque-rotational frequency characteristics of a driving motor. As shown in FIG. 2, the torque is maintained constant over the range of rotation frequencies 0-N.sub.1, whereas the output power becomes constant beyond the rotation frequency N.sub.1. In this figure, [1] indicates the characteristic curve when an amount of depression of an accelerator pedal is maximum, [3] indicates the characteristic curve when it is minimum, and [2] indicates the characteristic curve when it is intermediate.
The following requirements must be met in order to have electric vehicles used as often as internal combustion engine cars.
(1) Having good acceleration characteristics. PA1 (2) Having high mileage per charge. PA1 (3) Providing high reliability and ease of maintenance. PA1 (4) Comprising a battery of good performance in both output density (W/kg) and energy density (Wh/kg). PA1 (5) Comprising a mechanism that is highly efficient, small in size, light, and easy to maintain. PA1 (6) Low cost. PA1 (1) Having high efficiency, particularly in a low output range. PA1 (2) Providing large output power in acceleration. PA1 (3) Having a wide speed range. PA1 (4) Small in size, light, and inexpensive. PA1 (1) It can achieve a great output torque in acceleration. PA1 (2) It should have high total efficiency in a low output power range. PA1 (3) It should have high availability of a battery, thereby reducing the size and weight of the battery. PA1 (4) It should be of low cost. PA1 the AC motor comprising: PA1 wherein the inverter supplies the first and second stator windings with AC power independently, thereby driving the synchronous motor and the induction motor separately. PA1 (1) .tau..sub.s *=.tau.*, and .tau..sub.i *=0, when .tau.*.ltoreq..tau..sub.smax ; and PA1 (2) .tau..sub.s *=.tau..sub.smax, and .tau..sub.i *=.tau.*-.tau..sub.smax, when .tau.*&gt;.tau..sub.smax. PA1 (1) when .tau.*.ltoreq..tau..sub.scont, .tau..sub.s *=.tau.*, and .tau..sub.i *=0; PA1 (2) when .tau..sub.scont &lt;.tau.*.ltoreq..tau..sub.scont +.tau..sub.icont, .tau..sub.s *=.tau..sub.scont, and .tau..sub.i *=.tau.*-.tau..sub.scont ; PA1 (3) when .tau..sub.scont +.tau..sub.icont &lt;.tau.*.ltoreq..tau..sub.smax +.tau..sub.icont, .tau..sub.s *=.tau.*-.tau..sub.icont, and .tau..sub.i *=.tau..sub.icont ; and PA1 (4) when .tau.*&gt;.tau..sub.smax +.tau..sub.icont, .tau..sub.s *=.tau..sub.smax, and .tau..sub.i *=.tau.*-.tau..sub.smax. PA1 a first battery; PA1 a second battery; PA1 an AC motor for driving wheels of the electric vehicle, the AC motor including a synchronous motor and an induction motor; PA1 a first inverter connected between the first battery and the synchronous motor; and PA1 a second inverter connected between the second battery and the induction motor. PA1 an AC motor including a first rotor which has a permanent magnet and is mounted on a rotor shaft, a second rotor which is mounted on the rotor shaft, and first stator windings and second stator windings which are disposed in a manner such that they do not magnetically interfere with each other, and that they correspond to the first rotor and the second rotor, respectively, the first rotor and the first stator windings constituting a synchronous motor, and the second rotor and the second stator windings constituting an induction motor, the synchronous motor and the induction motor being integrally constructed into one body; PA1 an inverter supplying the first stator windings and the second stator windings with AC power, independently; PA1 first computing means for computing a rotation speed of the first rotor on the basis of actual values of voltages and currents of the synchronous motor or on the basis of command values of voltages and currents of the synchronous motor; and PA1 control means for controlling the induction motor by using the rotation speed computed by the first computing means as a feedback value corresponding to the speed of the induction motor. PA1 an AC motor including a permanent magnet synchronous motor and an induction motor which are integrally constructed into a single body, the synchronous motor having a rotor including a permanent magnet, and the synchronous motor and the induction motor having a common axis of rotation joined to a shaft for driving one or more wheels; PA1 a first inverter supplying AC power to windings of the synchronous motor; PA1 a second inverter supplying AC power to windings of the induction motor; PA1 a main battery supplying the first inverter and the second inverter with a DC voltage; PA1 first disconnecting means for electrically disconnecting the first inverter from the main battery; PA1 second disconnecting means for electrically disconnecting the second inverter from the main battery; and PA1 third disconnecting means connected to AC output lines of the first inverter for electrically disconnecting the synchronous motor from the first inverter.
FIG. 1 shows a typical system of such an electric vehicle, which employs a lead acid battery or nickel-cadmium cells as the battery 1, a brushless AC motor as the motor 5, and a transistor inverter as the inverter 4.
Next, the AC motor for the driving wheels will be described in more detail.
First, let us suppose that an induction motor is used as the AC motor 5.
As is known in the art, an induction motor generates its main magnetic flux from the primary current, and allows the magnetic flux and the torque to be independently controlled by a vector control. Thus, by employing a magnetic flux weakening control, the induction motor can provide a wide range of speeds in the driving system of an electric vehicle, in which the primary voltage is restricted by the voltage of the battery.
However, since the induction motor generates torque by inducing a current to flow through the secondary side, its efficiency reduces owing to the copper loss at the secondary side. This requires a large capacity cooling device. In addition, there is another problem in that the input efficiency of the motor reduces because the exciting current is supplied from the primary side and this increases copper loss and eddy current loss. In particular, the efficiency is greatly reduced in a low output power range, and this presents a great problem in the field of the electric vehicle.
Next, let us suppose that a synchronous motor is used as the AC motor.
Synchronous motors are generally divided into a revolving-armature type and a revolving-field type, both of which employ slip ring brushes or a rotary transformer in order to supply currents to the rotor windings. This, however, not only increases the size of the motor, but also reduces the efficiency thereof. Accordingly, a permanent magnet synchronous motor whose rotor is made of permanent magnets, and which is widely used as an AC servo-motor, is suitable for an electric vehicle.
This motor makes it possible to increase the power factor because it has no secondary copper loss, and hence provides high efficiency.
The permanent magnet synchronous motor has a constant field flux generated by the permanent magnets. In addition, the number of turns of the primary windings of the motor cannot be increased beyond a certain number because the voltage of the power supply is limited in the electric vehicle. Accordingly, it is very difficult for the motor to increase the rotation frequency and to provide required output power without increasing its currents. In other words, it is difficult to achieve high speed and large output power simultaneously under the condition that the currents are restricted to a certain amount.
Furthermore, high performance magnets that are used as permanent magnets are generally expensive, and hence the total cost of the system increases.
In Summary, requirements for AC motors for driving wheels of an electric vehicle are as follows:
Next let us consider the battery.
Although there are various types of batteries for an electric vehicle as mentioned above, there is no battery, for the present, that satisfies the output density (W/kg) and the energy density (Wh/kg) at the same time at reasonable cost. Accordingly, the type and capacity of the battery is decided considering the performance of the car, cost, and the like.
As an inverter, a transistor inverter is mainly used. This is because it is enough for the inverter for an electric vehicle to have a capacity not more than one hundred kVA, and an input voltage range of 100-300 V. The maximum output of the inverter takes place during acceleration, and in this case, the output current of the inverter reaches several hundred amperes. Thus, a plurality of power transistors are usually connected in parallel in the inverter.
FIG. 3 shows an example of a conventional AC variable speed driving apparatus using an AC motor and an inverter.
In this figure, a main circuit comprises an AC power supply 101, an inverter 102, an AC motor M, a speed sensor 12, and a position sensor 12'. The inverter includes a rectifier portion that performs AC/DC conversion, and an inverter portion that performs DC/AC conversion.
A control circuit, on the other hand, comprises an adder 103, a PI (Proportional-Integral) controller 104, and a voltage-current computing circuit 105. The adder 103 computes a speed difference .DELTA.n from an actual speed value n, which is detected by and fed from the sensor 12, and a speed command value n*. The PI controller 104 produces a torque command .tau.* such that the difference .DELTA.n becomes zero. The voltage-current computing circuit 105 computes from the torque command the voltage or current applied to the stator windings of the AC motor, and supplies it to the inverter 102 as a command value. In FIG. 3, the voltage-current computing circuit 105 provides the inverter 102 with a current command value i*.
The operation of the voltage-current computing circuit 105 varies in accordance with the type of the AC motor and the motor control scheme. When a permanent magnet synchronous motor is employed as the AC motor and the vector control like that used for an AC servo motor is adapted, the phase of the current whose amplitude is proportional to the torque command is made perpendicular to the position of the permanent magnet detected by the position sensor 12'.
On the other hand, when an induction motor is used as the AC motor, a vector control system as shown in FIG. 4 is widely employed. Details of the vector control system is described in 4th edition of "An AC servo motor and control of the same by a microcomputer" Sogou Denshi Publishing Ltd., Japan, Jun. 10, 1989, from which FIG. 4 is cited.
In the conventional system, the sensors 12 and 12' are required to detect the rotation speed of the AC motor and the magnetic position of a rotor. The sensors may sometimes cause faults in the system, and increases cost.
In view of this, various systems are proposed which drive a synchronous motor or an induction motor without using sensors. These systems, however, require a complicated control circuit and a complicated computing circuit.
In summary, the electric system of an electric vehicle must meet the following requirements.